Wave transmission



Ju y 28, 1942.

H. NYQUIST WAVE TRANSMIfiSION Filed June 5, 1940 2 sheets-sneer L FIG. 2

fifties X IMPEDANCE DISCO TINU/TY v 2-0 TIME 0R DISTANCE FIG. 6'

' INVENTOR rroRza-v tance between the two discontinuities.

Patented Jul 28, 1942 WAVE TRANSMISSION Harry Nyquist, Millburn, N. 1., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application June 5, 1940, Serial No. 338,889

13 Claims.

This invention relates to electric signal trans-,

mission systems and more particularly to the measurement and reduction of deleterious effacts due to repeated partial reflection of the signals at spaced points in the transmission path. It is well known that in long wire line signal transmission systems the line is usually not electrically smooth; that is, electrical irregularities or impedance discontinuities appear at many points along the line. These discontinuities may appear at signal repeater stations in the line or at any other point where electrical apparatus is connected to the line conductors, and they may appear to a lesser degree at splicing points and even within splicing lengths. At each such discontinuity some of the signal power is re- 'flected and proceeds back towards the signal source until it is blocked," as by the output circuit of a unidirectional signal repeater. The reflected power may on its way strike another impedance discontinuity'whereupon some of the power is again reflected and proceeds in the direction'of the original signal but delayed by the time consumed in twice traversing the dis- The signal component thus delayed eventually reaches the receiver at the far end of the line where it manifests itself as a distinct echo, if it be long enough delayed, or as distortion or as a ghost image if the signals are television signals. Reflection effects of the general kind illustrated in the foregoing description mayall be designated as echo effects, moreparticularly as listener echo efiects, whether the signals are of audio frequency or not. To mitigate them is a principal object of the present invention.

Heretofore efforts have been made to eliminate or electrically smooth out the impedance irregularities giving rise to echo effects, or echo suppressors have been employed in speech transmission systems, where appropriate, to disable the transmission circuit momentarily during pauses in the conversation so that the echo cannot be heard. The first of these remedies entails a search for the discontinuities and complicated and expensive compensating networks at each discontinu ty; it is not suited for compensation of discontinuities that appear within a reel len th or splicing length of shielded cable; and it would require sacrifice of certain advantages accompanying intentional mismatch of impedsystem at such points and with such respective shoes at the input circuits of signal repeaters.

The second remedy is of limited applicability and is quite inappropriate for television transmission systems. In systems of the latter kind the problem of reduction or substantial elimination of echoes is an aggravated and increasingly important one because an echo need bedelayed less than a millionth of a second behind the signal for it to require attention.

In accordance with the present invention many of the disadvantages of prior art methods and means for reducing echo effects are largely avoided, and certain impedance discontinuities that could not practically be reached and corrected heretofore are effectively compensated. The invention'also permits impedance irregularities to remain in the system as, for example, where their presence may be desirable for other reasons or where their number makes individual correction quite impractical, and yet permits correction of the echoes to which they give rise. In general the invention provides for the generation of one or more synthetic echoes and for their .introduction into the signaling delay, amplitude, phase and frequency composition that .the synthetic echoes and echoes arising in the signaling system efiectively offsetor neutralize each other. More particularly, in accord-. ance withpreferred embodiments of the invention, a portion of the wave power is diverted from the signaling system at a suitable point and transmitted through an auxiliary transmission line or other transducer of substantial electrical length, from which it is then returned to the signaling system, delayed just long enough by its passage through the auxiliary transducer for it to meet an echo following the signal. amplitude and frequency composition of the wave power thus returned to the system is or can be adjusted accurately to match the same parameters of the echo to be neutralized and its phase canreadily be so adjusted that the original echo and the synthetic echo are in phase opposition whereby they neutralize or tend to neutralize each other. Any number of echoes can be neutralized in the same manner by withdrawing wave power from the auxiliary transducer at various points along its length such that the delay encountered in transmission to those points coincides with-or is otherwise critically related to therespective delays of the several echoes.

Various specific objects of the invention are attained in the preferred embodiment by such features as the following. First, the auxiliary transducer is made in the form of a transmission line 7 whereby the 'delay of the synthetic echoes can be precisely and substantially continuously adjusted The impedance discontinuities causing them. Fourth,

separate frequency discriminating transducers are associated with the auxiliar transducer so that the modification of frequency composition introduced may be adjusted to the several frequency characteristics of the principal echo-producing discontinuities. Fifth, the auxiliary line is designed to have a relatively low velocity of propagation whereby the necessary delay can be realized in a short length of line. Sixth, to obviate an impedance discontinuity at the junction of the auxiliary transducer and the signaling line, the junction is placed between stages of a signal repeater amplifier. Seventh, in a transmission line having a largenumber of signal repeaters spaced apart therein, the echoes generated in a plurality of repeater sections are all neutralized at one point along the line; and in accordance with a related feature, the spacing of the principal discontinuities is made the same in each repeater section so that the principal echoes coincide in time delay. Eighth, to reduce the required length of auxiliary line, the latter is connected between widel spaced points along the signaling line. These and other features, objects and advantages of the invention will appear more fully in the appended detailed description of several illustrative embodiments.

The practice of thev invention as above described and more especially the adjustment of the length of the auxiliary transducer is simplified by use of a device for continuously representing the echo-producing characteristics of the transmission system so that the effect of adjustments of the neutralizing apparatus can be readily ascertained.

In accordance with a preferred .embodiment of the latter phase of the present invention, periodic current pulses are transmitted through the signaling system where they give rise to like ourrent-pulse echoes. The latter are separated at a neutralizing point from the pulses producing them and there caused to appear on the screen of a cathode ray tube, where also will appear the synthetic echoes introduced into the system. As the effective parameters of the echo neutralizing line are adjusted, there are corresponding changes evident on the screen of the cathode ray tube and it may be seen whether a given adjustment is in the proper sense to neutralize a given echo, and whether and to what extent the flnal adjustment is eifective.

In the further exposition of the invention reference will be made to the accompanying drawings, in which:

Figs. 1 to 4 are diagrams illustrating schematically how echoes arise from impedance discontinuities in a typical transmission system;

'Fig. 5 illustrates in idealized form one embodiment of the invention;

Fig. 6 shows an actual combination in accordance with the invention;

Fig. 'l'shows a modified form of the invention in which the required delay of the neutralizing echoes occurs partly in the signaling line;

Fig. 8 shows an embodiment with provision for differently modifying the frequency composition of diiferent sets of synthetic echoes;

Fig. 9 features an auxiliary transducer that is in the form of an electrically long but physically short acoustic transmission line;

Fig. 10 shows schematically a preferred arrangement for obtaining a visual representation of the echo-producing characteristics of the signaling system; and

Fig. 11 illustrates a detail thereof.

Referring now more particularly to Fig. 1, I

there'is represented schematically a portion of a signaling system comprising a line for the transmission of signals S and two successive signal repeaters I0 and II spaced apart in the line. Within the repeater section a, few electrical irregularities or impedance discontinuities -in the signaling circuit are represented at points I to 4 and at these points there is partial reflection of the signal power. It is assumed that at the output terminals 0 of the first repeater In there is a substantial discontinuity and that at the input terminals 5 of the second repeater II there is no discontinuity and no reflection of the signal power. On passing from one repeater to the next the signal S strikes the successive discontinuities I to 4 and at each of these points a portion of the signal power is reflected and proceeds toward the repeater II). On reaching the large discontinuity II at the output terminal oi repeater III the several reflected portions are again reflected and they now proceed toward repeater II in the same direction as the main body of the signals. While these reflections take place the signal continues to advance in the direction of normal propagation, and the several doubly reflected portions of the signal are accordingly relatively retarded or delayed.

Fig. 2 shows diagrammatically the relation between the signal and theseveral echoes produced in the manner described with reference to Fig. 1. The echo I-II, produced by successive reflections at points I and 0, is delayed by the time consumed in twice traversing the distance between points 0 and I; the delay of echo 20 is similarly measured by twice the separation of discontinuities 0 and 2; and so on. The last echo 40 has a delay comparable with twice the electrical length of the repeater section. Assuming that i the velocity of propagation is substantially uniform throughout the repeater section, the respective delays of the several echoes may be represented in terms of distance, as indicated in Fig. 2.

If there is a substantial impedance discontinuity at the input terminal 5 of the repeater I I, also, as represented in Fig. 3, an additional set of echoes will appear. flected at the discontinuity 5 will return toward repeater I 0 and on its way strike the several intermediate discontinuities I to 4. At each such point some of the wave power is again reflected .and resumes propagation in the direction of the signal. Of the added set of echoes the one least delayed is that designated 5-4, and the one with greatest delay 50 is displaced behind the signal by twice the electrical length of the repeater section. In considering Figs. 1 to 4 it should be appreciated that the relative intensity of an echo is aflfected by line attenuation and that more particularly it tends to decrease as the distance between the pair of discontinuities giving rise to it increases. Its intensity depends also on the de- Thus the signal power re- Y the fabricating machine.

. gree of impedance mismatch at the significant discontinuities and its relative phase is determined by the sense in which the impedance changes at the discontinuities.

In a long transmission system-comprising many repeater sections the effect described with reference toFigs. l to 4 will tend to arise in each section. Although the total number of echoes in the system will therefore tend to increasein proportion to the number of sections, it is important to note that none of the echoes ordinarily significant will be delayed by an amount more than that corresponding to twice the electrical length of the longest repeater section. Although there may he, therefore, a large number of echoes superposed in the same time interval, the problem of neutralization in accordance with the present 7 invention is simplified at least to the extent that thefmaximum delay is not proportional to the length of the system.

In view of the considerations outlined hereinbefore, it may now be evident that in a system in accordance with applicant's invention it is desirable to observe several conditions in practice.

' First, the line should be as electrically smooth as possible near the ends of each repeater section or at least near that end where there is a substantial discontinuity at the repeater, for the relative intensity of the echoes arising in those portions of the line is least reduced by attenuation.- It will contribute .to the desired electrical smoothness where the line is a cable or otherwise prefabricated, if the various splicing lengths 3 making up the line are arranged in the same order in which they are taken from the reel, and if the reel or cutting lengths in turn are arranged in the same order in which they were taken from The facing line impedances, then, at the junctions of splicing lengths and of cutting lengths will .tend to be equal and the probability of a substantial 'impedance discontinuity is minimized. In view also 'of the special nature of applicant's invention it is desirable that the pairs of discontinuities giving rise to substantial echoes be equally spaced in successive repeater sections. Thus for specific example it is recommended that the electrical length of the cutting length adjacent a repeater be made precisely the same in all other repeater sections, andthat the. electrical lengths of successive repeater sections be precisely the same if the discontinuities represented at and in Fig.

3 aremarked enough to contribute a significant echo. With the spacing of discontinuities as here inrecommended the echoes produced in one repeater section will coincide in time delay with the respectively corresponding echoes produced in all other repeater sections, with the net result that the .total number of significant echoes to be neutralized will be substantially reduced.

Fig. 5illustrates a portion of a long wire line signal transmission system comprising a shieldedpair cable l2 and signal repeatersspaced apart therein, with provision in accordance with the invention for the mitigation of echo eifects. In

this example, as well as in theothers to follow,

it may be assumed for purposes of specific illustration that the signals transmitted through the system are television signals occupying th frequency rang'e lying between 50 and 2500 kilocycles persecond, thatthe velocity of signal propagation through the line is approximately 170,000

miles per second, and that the repeaters are spaced about five miles apart. It maybe assumed further that at the output terminals-of one of the repeaters ll there appears in addition to the signal a plurality of substantial echoes which it is desired to eliminate so that a signal free of echoes may be delivered-to an immediately adjacent terminal station or to anothersection of line for transmission ultimately to a remote terminal station. It'may be noted here that from a theoretical standpoint it is immaterial and not of controlling importance whether the synthetic or neutralizing echoes introduced in accordance with the invention be introduced at a point in the line following the discontinuities giving rise to-echoes path l3. similar to cable I: and its electrical length is at but for-various reasons it is convenient and advantageous to do so. It should be borne in mind,

however, that it is quite feasible to introduce the, synthetic echoes at the point where the signals are first applied to the cable. 1

At the output terminalsof repeater I l in Fig. 5 a portion of the signal power is diverted to a sig-- nal amplifier 8 and caused to propagate corresponding signal waves through a transmission The latter may be a. shielded circuit least as great as the spaceor distance delay of the most retarded echo tobe neutralized At its remote end the auxiliary line I3 is terminated in an impedance matching resistor I so that there is no wave reflection'at the remote end. At various points along line 13 wave power 'is divertedthrough resistors it to a pair of bus-bars l5 from which they are. applied through amplifier "I to the line l2 at the output of repeater II. The busbars l5 may be assumed to have substantially zero elect'ricallength, a condition thatcan berealized in practice as will appear from examples presented hereinafter. The position of any given resistor I6 along the line I3 is so adjusted. that the time required for the signal to travel from the output of repeater l l to the given resistor coincides with the time delay of a given echp to be neutralized. The magnitude of the resistor is so adjusted, that the intensity of the synthetic echo introduced into the line through amplifier ll is'equal to that of the given echo to be neutralized. The phase of a given synthetic echo can be readily changed 180 degrees by connecting its associated resistor 18 with one orthe other of I the bus-bars l5.

One ay ordinarily dispense with the amplifiers 8 and 1 in Fig. S-oradjust them to. zeroor subzero gain. If they are omitted separate provision should be made for adjusting the amount of signalpower diverted from th line I 2 and for avoiding or minimizing an impedance discontinuity at the point of connection of line l3 and bus-bars I5. Account should be taken too of'th'e 1 able. In the illustrative case for which numerical data are given hereinabove sufficient accuracywill be achieved if taps are available on the neutralizing line l3 at points spaced apart a quarter or a sixth of the wave-length at the highest signal.

frequency. Under the conditions assumed the wave-length at 2500 kilocycles is 360 feet, and if the velocity of propagation in the lines l2 and I3 is equal, the indicated one-sixth and one-quarter of a wave-length would be 60 feet and 90 feet, respectively.

The practical illustrative embodim'entof the inf vention shown in Fig. 6 can be accommodated together, close enough that they can all be conlength that would be required inaccordan iywlt Fig. 6. If there are present in the signaling line echoes having delays too short to be matched with the Fig. 7 arrangement an auxiliary neutralizing line-in accordance with Fig. 6 may be connected at repeater In or II, for example, or a shorter backwardly extending neutralizing line in accord- ,ance with Fig. '7 may be added at any convennected by electrically short leads to a common terminal board, for example. The several taps can then, be connected through resistors l6, as

required, to bus-bars l5, which in turn are connected back to the signaling circuit 22. Pr'eferably the line 1: is connected to the circuit 22 at a point between successive stages and 2| of the repeater amplifier, and the bus-bars ii are likewise preferably connected to such a point so ient point to compensate for these echoes of rela tively short delay. It should be understood that the-principles illustrated in Fig. '7 are not limited in their application to the typical case assumed in which a relatively short repeater section is included in the system.

Y It was intimated hereinbefore that in a given transmission system the echoes might be found to be divisible into a plurality of sets, the echoes in each set evidencing in common the same fre-. quency' distortion relative to the signals. this condition may arise can be seen readily on reference to Figs. 3 and 4. If the impedance that a reflection-producing discontinuity will not appear at their point of connection to theline.

This feature may be employed in any of the. em-

bodiments herein disclosed. Where the interstage circuit to which bus-bars are connected is unbalanced or grounded on one side, the phase reversing function described with reference to Fig. 5 may be retained by coupling bus-bars l5 to the balanced circuit terminals of a balancedto-unbalanced transformer II, the unbalanced side of which is cqnnectedto the interstage circuit. The line l3 ,may, if desired, be buried or housed in a repeater station, or enclosed within an individual hut. Bus-bars I! may be shunted by a resistor l8, if desired, to minimize the effects of bridged capacities across the balanced side of the transformer, and particularly to'minimize the diflerence in the effects thereof at difierent frequencies. H

Instances ay be encountered in practice where a number ofrepeater sections of average length are followed by a substantially shorter one. In such instances it may be found advantageous to utilizethe modification of the invention that discontinuity at point I is v of the same degree and character at all frequencies in the signaling range, the echoes due to its presence will tend to be free of frequency distortionl If this principal impedance discontinuity, however, is such that there is a greater mismatch at some fre quencies, as for example the lower frequencies in the television range, than at other frequencies,

then the different frequency components of the echoes will not be of the same relative intensity asthe corresponding components in the signal, and frequency distortion may be said-to be present. The other principal discontinuity, that at I point 5, may have a different reflection-fre- .quency characteristic corresponding, for example, to a marked discontinuity at the upper television frequencies. To modify correspondingly the several sets of synthetic echoes so that is illustrated in Fig. 7 where a short repeater section bounded by repeaters Ill and H is assumed to follow one or more longer repeater sections. The neutralizing line 23-l 3 is indicated as being connected to the main transmission line at the output terminals of the second repeater II and as extending parallel to the main circuitto a point adjacent repeater I where the neutralizing line portion [3 is coiled in the manner illustrated in Fig. 6 and terminated. in an impedance matching resistor ll. "The coiled portion of the line I3 is provided with taps from which neutralizing connections comprising resistors l l are taken oi! and connected with the input circuit of repeater III.

The resistors ii are 50 adjusted and connected to such t'aps, as described hereinbefore, as to neutralize echoes appearing in the line. The neutralizing circuit is virtually unilateral in its operation by reason of its connection between a point of high signal level at repeater II and a point of low signal level at repeater Hi. It is to be observed that in Fig. 7 the delay of a given synthetic echo relative to the signal is measured by the combined lengths of the portion of the neu-' tralizing circuit 23-l3 through which it is transmitted and the length of the repeater section their frequency composition will match that or the echoes to be neutralized, they may be segregated in the'neutralizing circuit and respectlvely, corresponding frequency distortion 1m pressed on them.

Fig. 8 will serve to illustrate one arrangement for introducing frequency distortion in a plurality of sets of synthetic echoes. Although it is specifically adapted for'only two such sets of echoes, it will be evident that the principles in-. .volved canbe extended to accommodate-any desired number of sets of echoes. In Fig. 8 a neutrallzing line I: isconnected to the signal line in the manner of Fig. 5 and two sets of bus-bars l5a'and lib are associated therewith instead of the single set indicated in the preceding figures. As in Fig. 5, the bus-bars are idealized inthat they are treated as being electrically short. Connecting each pair of bus-bars with the neutralizing' line I3 is an individual set of resistors [6a, 16b, which are proportioned and positioned to neutralize-a corresponding set of echoes in the,

, to the two sets of bus-bars are separately and between repeaters l0 and II.- By reason of this fact a given delay is achieved in Fig. '7 with a neutralizing line that is little more than half the differently treated in the respectively corresponding equalizers 25 and 26 before they are introduced into the signaling line. Each equalizer is designed to impress on the set of synthetic echoes applied to it the same modification, as of frequency distortion, that appears in the set of echoes they are intended ,to'neutrali'ze. To avoid interaction between the two equalizers and their associated circuits they may be connected in conjugate relation to each other by a hybrid coil 21 which connects them both through a circuit 28 in energy transfer relation with-the signaling line. 7 r

uniform electric transmission line,

and especially one having the same propagation A characteristics as the signaling line, is peculiarly .Although a takes the illustrative form of an acoustic wave guide or speaking tube 3. At one end of the tube is an acoustic wave generator 30 which is excited by signal power diverted from the signaling line. At its other end the tube- 3 is terminated in characteristic impedance by means of acoustic wave power absorbing material H4.

Acoustic wave receivers 3| spaced along the tube 3 constitute taps through which wave power may be diverted as required, through respective resistors H6, to a pair of bus-liars 5. It will be clear that a series of sound wave generators used in the location of receivers 3| and a receiver used in the location of sound wave generator 30 would constitute an alternative embodiment.'

The syntheticechoes introduced into the signaling line by the bus-bars 5 are adjusted in amplitude by control of resistors H6, and in phase and delay as described hereinbefore, to neutralize echoes appearing in the transmission line. The receivers 3| may be made continuously adjustable longitudinally of the tube 3 to facilitate accurate control of the delay of the synthetic echoes. For signaling waves of speech frequency the acoustic wave generator 30 and the receivers 3| may comprise conventional telephone apparatus, but for higher frequencies and wider-transmission bands other devices such as piezoelectric crystals are tobe preferred.

It is conceivable that satisfactory adjustment of theecho' neutralizing line in any of the foreo n embodiments might be effected by concurrent inspection, aural or visual, of the signal, and

that in the case of a television system one could employ a fixed signal pattern especially designed to disclose the presence and other information relating .to unneutralized echoes arising in the system. Preferably, however; adjustment of the end pulse generator 46 similar to pulse generator -42 and excited from synchronizing line 4| may be provided and connected to neutralize the original signal pulses. Neutralization may be effected by means of a hybrid coil 48 with the aid of a phase shifter 41 in the exciting circuit of pulse generator 46. With these features present only the pulse echoes are applied to the deflecting plates of the cathode ray tube and only the pulse echoes appear in the image.

If the pulses generated in device 42 are unidirectional, the image appearing on the screen of the cathode ray tube 43 may take the form suggested by the solid line pattern in Fig. 11. If the pulses are of successively opposite polarity the image will be duplicated in reverse as suggested by the dotted line in Fig. 11. It will be evident that in either case the image indicates the'number of echoes, their relative amplitude and relative delay. On connecting to one of the taps of the neutralizing line l3 associated with repeater H, a synthetic echo will ordinarily appear in the image. By changing the position of the tap the synthetic. echo image may be caused to move across \the cathode ray tube screen until it coincides in time delay with-one of the. original echoes. Further adjustment is then made of its amplitude and phase until both substantially disappear. each significant echo present.

. With regard. to the wave source 40, in Fig. 10 considerable latitude is permissible as to its operating frequency. What-frequency will be found the best will depend on the character of the pulses generated by-device 42. The p lses may be, for

example, of great intensity but of short duration, even small compared with the semiperiod of the highest normal signaling. frequency. This type of pulse is well. adapted'for exploring thechara'cteristics' of the 'system' in" the; upper frequency ranges. A square top wave of greater duration,

' n the other hand, is better adaptedfor the lower portions of the signaling range. If-the noise level on the transmission line is high, improved results may be obtained by rounding off the pulses applied to the line. A band-pass filter 49 in'the output lead from hybrid coil 48- may be designed neutralizing line is effected with the aid'of a con tinuous representation of the echo-producing characte'ristics'of the line. A preferred arrangement is illustrated in Fig. 10. g

In Fig. 10 it may be supposed that the echo-- producing characteristics of the portion of sigline 22 periodic short current pulses. As each signal pulse and its attendant train of pulse echoes arrive at the echo neutralizing point they are diverted from the signaling line and at least the pulse echoes made to appear on the screen of a cathode ray tube 43. -The sweep circuit 44 associated with the cathode ray tube is synchronized with the incoming pulses by means of a circuit 4|. which connects it with the source 40. A phase shifter 45 may be associated with the sweep circuit to control the position of the image on the screen. When the system is properly synchronized the image of the echoes remains stationary. To eliminate the signal pulse from the circuits of the cathode ray tube a sec- :to introduce a'supplementary rounding ofi of.

the'signal pulses and at the same time to exclude frequencies lying outside the range for which it is desiredto provide echo neutralization.

' Although the present invention has been de-, scribed largely in terms of various illustrative embodiments, it will'be apparent to those skilled in the art that the invention-and its various features are susceptible of embodiment in a wide variety of forms within the spirit and scope of the appended claims. What is claimed is:

1. In combination, a signal transmission line,

an auxiliary uniform transmission line connected at two geographically separated points'thereof in signal transducing relation with said signal transmission line, the total electrical length of the loop formed by said auxiliary line and a portion of the said signal transmission line embraced by said.

two points being substantally equalto the distance delay of asignal echo'arising in said line.

2. In combination with a long distance signal.

transmission line subject to echo effects, an auxiliary transmission line the electrical length of which-is comparable with the delay of echoes in said line, means for exciting said' auxiliary line with signals transmitted through said signal transmission line, a plurality of spaced'taps along The same procedure is repeated for said auxiliary line for deriving synthetic echoes irom said auxiliary line, andmeans introducing the echoes so derived into said transmission line .ior the neutralization of echoes "arising therein,

said auxiliaryline being coiled or otherwise so disposed that said taps are physically close to- 1 ther. 3. A combination in accordance withiiilaim 2 in which the propagation constant of said auxiliary line is substantially identical with the propagation constant-of said signal transmission line.

4. In a signaling system, a long distance transmission line comprising a plurality of successive repeater sections, each of said sections having therein a plurality'ofspaced impedance discon-- tinuities that give rise to signal echoes, means at one point along said line for introducing into said line a plurality of synthetic echoes to neutralize the said. signal echoes, the pairs of discontinuities in each repeater section that give rise to substantial echoes being equally spaced in all 01 said sections, the equality of spacing being of such precision that respectively corresponding echoes arising in said several sections efiectively coincide for echo neutralizing purposes.

5. The method of'mitigating echo efiects in a signal transmission system which comprises maintaining the echoes arisingin successive sections of said system in superposed relation and introducing into said system synthetic echoes sev- I point that the respective delays of said waves so introduced are at said point substantially equal to the delays of respectively corresponding ghost images.

.7. In combination, a signal transmission hne having a plurality of, signal repeater geograph ically spaced apart therein, and an auxiliary signal transmission line connecting a point of high signal level at one repeater with a point of comparatively low signal level at an interior repeater, said line being so connected as to divert signal waves from said line at said one repeater and to introduce the diverted'signal waves into said lin atsaid anterior repeater, the combined electrical lengths of said auxiliary line and the portion of the signaling line embraced thereby being substantially equal to the delay of a signal echo appearing in said transmission line.

8. In combination, a signal transmission path comprising a plurality of signal repeaters 'geographically spaced apart therein, said repeaters defining a plurality of successive repeater sections of approximately equal electrical lengths followed bya substantially shorter repeater section, and means for introducing synthetic echoes into said signal transmission path comprising an auxiliary signal transmission link embracing said shorter repeater section and feeding signal waves back to the input end of said shorter section.

9. A combination in accordance with claim 8 in v which said auxiliary link comprises a transmission line that is coiled at said input end of said shorter section, and electrically short signal transducing connections from a plurality of points along said coiled line to said transmission path.

l0. In combination with a signaling system, an electrically long signal wave transducer excited by signals traversing said system, means for deriving separate sets of differently delayed synthetic echoes from said transducer, means 'for modifying the frequency composition of at least one of said separate sets, and means introducing said separate sets after modification into said signaling system.

11. A long distance signal transmission path comprising a plurality of signal repeaters geo-' graphicallyspaced apart therein, an electrically long wave guide excited at one end by signals from said transmission path, an impedance matching termination for said guide at the other end thereof, a plurality of sets of taps for divert ing variously delayed synthetic echoes from said guide, an individual equalizer for each of said sets of taps, and means for introducing the corresponding sets of synthetic echoes into said transmission path through said individual equalizers, said synthetic echoes having such delay as to coincide with echoes arisingin said transmission path, and said equalizers having such respective transmission characteristics that each-of said sets of synthetic echoes is modified with respect to said signal in' substantially the same mannor as is a respectively corresponding set of said echoes arising in said transmission path.

12. In a long transmission system subject to echo effects, the method which comprises transmitting periodic discrete pulses throughsaid sys tem, receiving said pulses and the correspondingly periodic sets of echoes produced by said pulses, neutralizing the said received pulses andtranslating said sets of echoes into a visual pattern,

whereby the echo-producing characteristics of said system may be observed in said pattern, introducing synthetic pulse echoes into said system at a point such that said synthetic echoes are concurrently received and translated as aforesaid, and adjusting the delay of said synthetic echoes as introduced into said system so that said pattern assumes a predetermined character indicative of substantial neutralization of said echoes. 13. A long distance signal transmission system subject to echo effects, means for introducing a synthetic neutralizing echo into said system comprising an auxiliary transmission path interconnecting geographically separated points in said system for the transmission of signals therebetween, the total length of the loop vformed by said auxiliary path and the portion of said system between said geographically separated points being substantially equal to the electrical distance delay of anecho in said system, and means for adjustin the amplitude of said synthetic echo to substantial equality with that of the echo to be neutralized. HARRY NYQUIST. 

